Post stages existed in France (and elsewhere in Europe) before the Revolution
- where horses would be exchanged along the route
The Wikipedia article Stage station deals more about the 19th century than the 18th century, but otherwise gives a good overview on how the system worked.
It also explains the Origin of the name posting:
Medieval couriers were caballari postarus or riders of the posts. The riders mounted fresh horses at each post on their route and then rode on. Post came to be applied to the riders then to the mail they carried and eventually to the whole system. In England regular posts were set up in the 16th century.
The riders of the posts carried the government’s letters. The local postmasters delivered the letters as well as providing horses to the royal couriers. They also provided horses to other travellers.
It was intended not only for carriages, but also for individual travelling and for messengers.
The messenger would start in Paris and regularly change horses on the way to Calais
- the greatest danger was getting a lowsy horse in exchange
So a messenger could, for the time, travel swiftly between
- Paris -> Calais
- Dover -> London
the unknown factor was then only the weather and tide conditions for crossing the Channel using the packet boats or other services that may have been available.
Tour from London to Paris (July 1790) gives the crossing times as
- about 3 hours, with a rough passage (Dover -> Calais)
- 5 hours, 11 PM to 5 AM (Calais -> Dover)
- plus 2 hours waiting for the tide in Calais
- due low water in Dover
- last half mile on an extra boat was needed
- arrival in Calais at 4 PM
- left Dover for London 12 noon
From the History of Transport and Travel:
Carriages: 17th century
Coaches gradually become more comfortable. The most common design, developed in Germany in about 1660, is known as the berlin. The compartment for the travellers has the shape of a shallow U, with a protective roof above. There is a door on each side and the coach can seat four people, in pairs facing each other. The coachman, driving the horses, sits above the front wheels.
From 1680 glass windows keep out the weather, where previously there were only blinds. The first simple suspension, protecting the occupants against the bumps of the road, consists of leather straps on which the compartment hangs from the framework. The berlin introduces curved metal springs, which absorb the shocks more effectively.
Following the aborted trip to Saint Cloud, von Fersen revived these plans with vigor. In June, he acquired a Berline and drove it to a courtyard at Eleanore Sullivan's residence on the Rue de Clichy in Paris. The escape was arranged to take place on 20 June, coinciding with a particular guard change.
Fersen had urged the use of two light carriages that could have made the 200-mile journey to Montmédy relatively quickly. This would have involved the splitting up of the royal family, however, thus Louis and Marie-Antoinette decided on the use of a heavy and conspicuous coach drawn by six horses.
Due to the cumulative effect of slow progression, time miscalculations, lack of secrecy, and the need to repair broken coach traces, the royal family was thwarted in its escape attempt after leaving Paris. Louis himself chatted with peasants while horses were being changed at Fromentieres and Marie Antoinette gave silver dishes to a helpful local official at Chaintrix. At Châlons townspeople reportedly greeted and applauded the royal party. Finally, Jean-Baptiste Drouet, the postmaster of Sainte-Menehould, recognized the king from his portrait printed on an assignat in his possession. Seven detachments of cavalry posted along the intended route had been withdrawn or neutralized by suspicious crowds before the large and slow moving vehicle being used by the royal party had reached them. The king and his family were eventually arrested in the town of Varennes, 50 km (31 miles) from their ultimate destination, the heavily fortified royalist citadel of Montmédy.
Stagecoach and post chaise: 17th - 18th century
Travel between towns by public transport, in the 17th and 18th century, is a slow business. The stagecoach, a heavy and cumbersome carriage often without any form of springs, is introduced in Britain in 1640.
Up to eight of the more prosperous passengers can be packed inside a stagecoach. Second-class seats are available in a large open basket attached to the back. The least privileged travellers sit on the roof with the luggage, relying on a hand rail to prevent themselves slithering off.
This immensely unwieldly vehicle, drawn by either four or six horses, lurches along the rutted roads at an average speed of about four miles an hour. Danger from highwaymen is only one of many inconveniences on such a journey.
Their travels throughout the English countryside by coach provide the chief theme of the novel. A distinctive and valuable feature of the work is the generally accurate description of the old coaching inns of England.
The noble and the rich, such as young men on their way through Europe on the Grand Tour, travel in greater comfort - in private, and in well-sprung upholstered carriages. Their favoured vehicle is the post chaise, introduced in France in the early 18th century. Its name accurately suggests a pleasant seat, and an expectation of lively new horses at each post stage during the journey.
From Tour from London to Paris (July 1790)
- Dover -> Calais : about 3 hours, rough passage
- French coinage (conversion rate to Sterling) and measurements
- Calais -> Paris four days and four hours
- stage coach drawn by 8 horses
- Chapter 3
- Paris -> London
- 89 hours, 'which is 19 hours short of time commonly allowed for the performance thereof'
- Chapter 24